1. Field
Embodiments of the present disclosure relate to assembly of threaded joints and joints so assembled. In certain embodiments, the present disclosure pertains to a combination of thread tolerances, position control, and coatings for use in assembly of threaded connections used to join pipes in oil and gas applications.
2. Description of the Related Art
In oilfields, it is common practice to use metallic pipes to extract oil or gas from underground reservoirs or pools. Extraction may include boring a well into the earth and internal casement of the well. Casement provides the well with structural stability, inhibiting collapse of the well, and may be performed using a string of relatively large diameter metallic tubes, referred to as the casing. After the well has been bored to the desired depth and the casement has been put in place, a string of metallic pipes of smaller diameter may also be placed in the well. These smaller pipes, referred to as the tubing, may pump the gaseous or liquid hydrocarbons to the surface. The smaller pipes that form the tubing are joined in a string so to extend over a length sufficient to reach the depth of extraction. Both the casing string and tubing strings may be formed of pipe segments joined together using threaded connectors.
During assembly of either the casing pipes or the tubing pipes, threads and other surfaces of the pipes that are in sliding contact may experience galling. Galling refers surface damage (e.g., adhesive wear) arising from contact between sliding solids and is distinguished by localized roughening and creation of protrusions above the original surface. For example, asperities present on the respective sliding surfaces may experience high pressures, raising the temperature and adhesion within the contact zone of the asperities. With continued contact, material transfer between the surfaces may occur and the transferred material may grow into protrusions. If a protrusion of transferred material grows to a large enough height (e.g., several microns), the protrusion may cause damage to the opposing surface. Furthermore, the surfaces may experience increased adhesion and the friction needed for further advancement.
It is common practice to use dope or grease on the surface of the threads of male and female elements of the threaded connection during assembly of the pipes in order to inhibit galling. The grease or dope also plays a significant role in sealability by filling the space between crests and roots of the respective pipe threads when the connection is assembled, blocking paths for possible leaks (see, e.g., FIG. 1). The dope may include small particles of heavy metals, such as Pb or Cu.
The use of dope presents significant drawbacks, however. Notably, high concentrations of heavy metals can result in heavy metal accumulation in the bodies of humans and animals, causing serious illness. Furthermore, the heavy metals present in dope may contaminate soil, groundwater, and ocean water, presenting an environmental hazard. Furthermore, in light of these hazards, strict regulations prohibiting discharge of heavy metals may necessitate precautions when utilizing dope, increasing the cost to employ dope.
In addition to the aforementioned issues, the use of dope may further necessitate operations such as cleaning and doping of the threaded connections in the field during use. These operations are expensive and time consuming, as they are labor intensive. Additionally, these operations may expose personnel to safety risks, as they require exposure to moving pipes and often take place under adverse conditions.
Other issues encountered when employing dope may include either “underdoping” or “overdoping.” In underdoping, insufficient dope is provided to the threaded connection and/or the dope fails to distribute over the surface of the threaded connection. As a result, the likelihood of galling is increased within underdoped regions.
While application of excess dope may address the issue of underdoping, this approach risks overdoping, the condition of placing too much dope on the threads of the connectors. Overdoping may create conditions such that, during assembly of the connector, excess dope is unable to be evacuated through the end of the threaded portions of the pipe segments. The trapped dope can develop high pressure within the threaded connection and lead to plastic deformation of the pipe segments in the threaded portion. In extreme cases, this plastic deformation may cause collapse of the male member of the joint, rendering the connector ineffective, and requiring replacement of the pipe segments and/or the sleeve.
Furthermore, while dopes have been developed which substitute fillers for heavy metals, these dopes are also problematic. In one example, there are presently no standards for these fillers. As such, high friction variability and tribological behavior may be observed between different proprietary manufacturer formulations. This variability is undesirable as it may lead to uncertainty in joint performance between each brand of dope.